This invention relates to servo compensators, and more particularly pertains to an integrated circuit analog-signal co-processor having a digitally programmable circuit configuration and a digitally programmable transfer-function.
Traditional approaches to servo system design use either analog signal processing or digital signal processing for establishing the servo compensator transfer function, or control law. The elementary servo system of FIG. 1 includes a servo compensator 10, motor 12, motion and/or position mechanical to electrical transducer 14 and a summer 16. The input command signal is most often an analog signal and when the compensator is of the digital signal-handling type, the input command line must include an analog-to-digital converter.
There has recently been seen a dramatic increase in the complexity of servos, especially for computer disk memory drives. And, the pressure for improved disk drive performance has notably increased, e.g. demand for high speed, without giving up accuracy, degrading versatility or diminishing reliability.
Servo compensators include at minimum a filter circuit for "shaping" the response of the motor control signal. But servo compensators are usually used in conjunction with a lead or prediction circuit. A commonly used lead or prediction circuit is a proportional-derivative (PD) compensator circuit or a proportional-integration-derivative (PID) compensator circuit to account for the inertia and momentum of the motor toward greater speed and accuracy. Many times the compensator also includes a notch filter of center frequency equal to a principal mechanical vibration frequency of the motor-load.
One recent refinement has a simple microprocessor that in response to the measured parameters of the motor-load, status will program the compensator filter's frequency response to optimize control loop performance. The compensator is then thought of as a servo co-processor compensator.
Servo co-processors of the prior art, with the exception of the motor driver, have been entirely implemented by digital circuity whereby both the compensator and its microprocessor-associated programming circuits are all built using the same compatible technology and processes, e.g. CMOS. However, it becomes difficult and expensive to achieve analog-signal sampling rates higher than about 100 KHz, partly because of the delays introduced by the required analog-to-digital converters (ADC's) through which the analog command signal is introduced to the compensator, and partly because digital arithmetic operations are time consuming. Digital-signal processors typically have a clock rate of 2 KHz.
It is therefore an object of this invention to provide a faster and more broadly competent integrated-circuit servo-compensator co-processor that is also capable of providing high accuracy and stability, especially for use in disk drives.